Need help in understanding delegated permissions for onlinemeetingtranscript.read.all

Hi everyone,

I am building a custom team app which is basically a meeting bot which retrieves the transcripts of the meeting when the meeting ends. I followed this Microsoft sample
https://github.com/OfficeDev/Microsoft-Teams-Samples/tree/main/samples/meetings-transcription/nodejs

There are some permissions(OnlineMeetingTranscript.Read.All) needed to use the users/{user_id}/onlinemeetings/{meeting_id}/transcripts api.

These permissions can be delegated level or application level as per the documentation.
https://learn.microsoft.com/en-us/graph/api/onlinemeeting-list-transcripts?view=graph-rest-beta&tabs=python

The API works fine with application level permissions.

But when I try to make it delegated, I use the
me/onlinemeetings/{meeting_id}/transcripts api.

the api returns a 400 Bad request error

I wanted to understand if a delegated level of permission for my use case (meeting transcription bot) is feasible or not. 
Is it a hard requirement that the permission level should be application level for the meeting transcription bot? If yes, why?